6. Metals and metalloids of group 15 & 16. Properties and trends of metals and metalloids of group 15 & 16 Francisco Javier Cervigon Ruckauer

6. Metals and metalloids of group 15 & 16

Properties and trends of metals and metalloids of group 15 & 16

METALS AND METALLOIDS OF GROUPS 15 & 16

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PHYSICAL PROPERTIES OF ELEMENTS FROM GROUP 15 AND 16

In Table 6.1 some important physical properties of elements of group 15 are summarised. We have included nitrogen and phosphorus for comparison. As it can be noted as we going down the group the boiling point increases except for bismuth which has a lower value. The low melting point of bismuth and the even lower melting point of its alloys are very important for the major uses of this element. This low value can be attributed presumably to the weak forces of attraction between atoms in the solid state due to the tendency of bismuth to form three rather than five covalent bonds. Liquid bismuth also has the unusual property of expanding when it freezes, similar to what happens to water. Ionization energies for elements of group 15 decrease going down the group depending on the increase in size of elements. It is should be noted that elements from group 15 have much higher ionization energy values than that found for elements in group 14, which is attributed to their higher nuclear effective charge and the greater stability of their electronic configurations due to the half-filled p-orbitals of their valence shells. Finally, as appears in table 6.1, we can observe how with the increase of atomic number there is an increase of density as usually has been described in other groups.

Some relevant data are collected in Table 6.2 for elements of group 16. As expected melting and boiling points increase regularly as you go down the group, according to the higher molecular size and therefore higher Van der Waals Forces. However this trend is changed when we reach polonium which resembles its horizontal neighbors showing a remarkably lower melting and boiling point. That can be explained by the diminished availability of the s electron pair. Elements from group 16 show higher ionization potential energies than elements of group 15 due to the increase of effective nuclear charge, and these values decrease, as expected, down the group with the increase in size of the elements. These elements do not form monoatomic cations with the possible exception of 

Po

O2.

CHEMICAL PROPERTIES OF ELEMENTS FROM GROUP 15 AND 16

The main feature of the reactivity of elements of group 15 is that it is appreciably different from one element to another. Nitrogen, for example, is almost inert, mainly due to the high stability of the N2 molecule, while phosphorous is extremely reactive, especially in its white form. The rest of the elements are comparatively less reactive and although the reactivity is complicated the trends observed in previous groups are still apparent. If we examine in detail some of these trends we observe the following:

• All elements of group 15 form hydrides of general formula MH3 which are all covalent in nature. It can be observed that their thermal stability decreases down the group which can be explained in terms of their bond dissociation energy which is lower as you move down the group. For that reason, the reducing character of these compounds differing from one to another is well known. The reducing character ranges from the mild properties of ammonia to the strong character of AsH3 or SbH3. BiH3 is thermally unstable and decomposes above -45 ºC.
• All elements of the group combine with oxygen to form oxides of the type M2O3 and M2O5. The basicity of the oxides increases down the group and with respect to the oxidation state the oxides in higher oxidation states are more acidic. The most acidic oxides form stable oxoacids when dissolved in water.
• Elements of group 15 react with halogens to form two series of halides, trihalides and pentahalides. Trihalides are mainly covalent with the exception of BiF3 and therefore are easily hydrolysed in water. The product of hydrolysis differ from the first elements to the elements at the bottom of the group. For example if we examine the reaction of PCl3 and BiCl3:
PCl3+H2O→H3PO3+3HCl
BiCl3+H2O→BiOCl+2HCl
• As can be observed, the ease of hydrolysis decreases going down the group. Additionally, all elements of the group, except nitrogen, form pentahalides which are comparatively less stable than corresponding trihalides.

In the case of the elements of group 16, as was observed in the previous group, the first element, oxygen, has a distinctive chemical behavior due to its higher electronegativity, small radius and absence of accessible d orbitals. For example, the high electronegativy of oxygen allows it to form ionic compounds with metals, while this tendency decreases as you move down the group. Oxygen and sulphur are the most reactive compounds of the group although going down the group the reactivity decreases. Some important combinations are the following:

• All elements of the group form hydrides of the form H2E, (where E is the element of the group) and as was described for elements of group 15, the stability of these compounds decreases from H2O to H2Te. These hydrides are weakly acid in nature and the acid strength increases down the group. Hydrides of these elements are reducing agents with the exception of water. They are also volatile substances with the exception of water due to the molecular association of water molecules by hydrogen bonding. For the rest of hydrides, only Van der Waal’s forces are considered due to the absence of hydrogen bonding for these compounds, and accordingly volatility decreases from H2S to H2Te, as the size of the element increases.
• Elements of group 16 form a wide number of halides in different oxidation states, where combinations with the elements in the lower oxidation state are less stable for the heavier elements of the group.
• All elements form a wide variety of oxides with different stoichiometries showing different acid and base behavior according the element and oxidation state. For example, oxides of formula MO2 are acidic for the first elements of the group and as you move down the group this acid character decreases, and TeO2 is considered amphoteric. Acidic oxides combine with water to form oxoacids, where the chemistry of sulphur resembles the wide variety found for phosphorous, being simpler for the case of selenium and tellurium. Trioxides are more unstable for the heavier elements and PoO3 has been only detected on a tracer scale.

Francisco Javier Cervigon Ruckauer